Prosecutors in Egypt have doubled a 15-day detention period of a 15-year-old boy for having an anti-regime sticker on his school ruler and issued arrest warrants for two of his teachers and his father.

In one of a series of cases which have highlighted the measures the authorities will go to in their determination to end pro-Muslim Brotherhood opposition. The boy, Khaled Mohammed Bakara, was detained after his schoolteacher saw the sticker and reported him to the police.

The sticker showed a hand holding up four fingers - the sign used by supporters of the deposed Brotherhood president, Mohammed Morsi, since the crushing of protests against his removal at a cost of hundreds of deaths at the Raba’a al-Adawiya mosque in Cairo. “Rabaa” means “four” in Egyptian Arabic.

The “Raba’a” sign, normally in black on a yellow background, has been held up at many subsequent demonstrations.

Khaled was initially ordered to be detained for 15 days pending inquiries into a claim that he possessed a “ruler bearing a sign that implies violence and offends the armed forces”.

He has subsequently been ordered to remain a further 15 days in detention at the Central Security Forces barracks in his home governorate of Kafr al-Sheikh.

At the same time, prosecutors issued arrest warrants for two of his teachers, one of whom was a relative, and his father, Mohammed Abdulghani Bakara, a fisherman, for “inciting” Khaled to take the ruler to school, along with two notebooks found in his school bag which also carried the symbol.

“Khaled has confessed that the ruler is his own, so there is no need to keep him in jail without bringing him to court,” his lawyer, Amr Ali al-Deen said. “But the prosecutor is now afraid to bring the case to court as it will harm his reputation even more than his reputation has already suffered. The case doesn’t even rise to the level of a misdemeanour.”

The arrest was ordered despite commitments from the military-backed authorities to freedom of speech, enshrined in a new constitution signed by the interim president, Adly Mansour, last week.

The constitution will be subject to a nationwide referendum next month, though the Muslim Brotherhood-led “Anti-Coup Alliance” announced on Monday that it would urge a boycott of the vote.

Khaled’s father told The Daily Telegraph he was still waiting for the arrest warrant to be executed. “Khaled is just a child,” he said.

“There is no background to this. Khaled is a placid child and not a trouble-maker. The whole situation is weird. We are waiting to see how it will end. There is no case to answer, they didn’t find him with a gun or a knife. It was just a ruler and there is no crime in the Raba’a symbol.”